Balancing crystallinity and specific surface area of metal-organic framework derived nickel hydroxide for high-performance supercapacitor

M-Ni-Co MOF (M=Zn, Fe, Mn) for high-performance supercapacitors by adjusting its morphology

Metal-organic frameworks (MOF) have been wildly synthesised and studied as electrode materials for supercapacitors, and bimetallic MOF of Ni and Co has been broadly studied to enhance both specific capacitance and stability of supercapacitors. Herein, a best performance (about 320 F/g) of Ni-Co bimetallic MOF was found in a uniform preparation condition by adjusting the ratio of Ni to Co. Then tiny third metal ion was introduced, and we found that the morphology of material has a significant change on the original basis. Furthermore, certain ions (Zn, Fe, Mn) introduced make a huge improvement in capacitance based on Ni-Co MOF of 320 F/g. The result shows that Zn-Ni-Co MOF, Fe-Ni-Co MOF and Mn-Ni-Co MOF perform specific capacitance of 1135 F/g, 870 F/g and 760F/g at 1 A/g, respectively. Meanwhile, the asymmetric supercapacitor (ASC) was constructed by Zn-Ni-Co MOF as positive electrode and active carbon (AC) as negative electrode. The Zn-Ni-Co MOF//AC ASC possesses a energy density of 58 Wh/kg at a power density of 775 W/kg. This research provides a new methods to regulate the morphology of MOF and a novel viewpoint for assembling high-performance, low-price, and eco-friendly green energy storage devices.

Metal-Organic Framework Materials for Electrochemical Supercapacitors

Exploring new materials with high stability and capacity is full of challenges in sustainable energy conversion and storage systems. Metal-organic frameworks (MOFs), as a new type of porous material, show the advantages of large specific surface area, high porosity, low density, and adjustable pore size, exhibiting a broad application prospect in the field of electrocatalytic reactions, batteries, particularly in the field of supercapacitors. This comprehensive review outlines the recent progress in synthetic methods and electrochemical performances of MOF materials, as well as their applications in supercapacitors. Additionally, the superiorities of MOFs-related materials are highlighted, while major challenges or opportunities for future research on them for electrochemical supercapacitors have been discussed and displayed, along with extensive experimental experiences.

Conversion of Amorphous MOF Microspheres into a Nickel Phosphate Battery-Type Electrode Using the "Anticollapse" Two-Step Strategy

Metal-organic frameworks (MOFs) have attracted great attention as templates for preparation of functional porous materials owing to their adjustable structures, rich porosity, and controllable components. However, collapsed templates during the conversion process hinder their application and synthesis of derivatives. In this study, we demonstrate a novel two-step etching strategy during which amorphous MOF microspheres are initially transformed into nickel hydroxide and then subsequently transformed into microspherical nickel phosphates. Through this strategy, the prepared nickel phosphates maintain the microspherical morphology of MOFs but with no MOF residuals, exhibiting ultrahigh specific surface area, uniform pore size, and good structural robustness. Examined as a supercapacitor electrode, they show an outstanding specific capacity of 820 C g^-1 at 0.5 A g^-1 and remarkable cycling stability of 88% capacity retention after 10 000 cycles. Moreover, an asymmetric supercapacitor constructed utilizing reduced graphene cross-linked with p-phenylenediamine oxide (PPD-rGO) as the cathode displays a preeminent energy density of 64.56 Wh kg^-1 at a power density of 507 W kg^-1. This strategy has important significance in guiding the preparation of high-performance MOF-derived electrodes.

Improved ionic diffusion and interfacial charge/mass transfer of ZIF-67-derived Ni–Co-LDH electrodes with bare ZIF-residual for enhanced supercapacitor performance

The crystallization of Ni–Co-LDH and the morphology of the hierarchical structure can be simply optimized to achieve improved ionic diffusion and reduce reaction resistance of charge/mass transfer between the electrode/electrolyte interfaces.

Amorphous nickel sulfide nanoparticles anchored on N-doped graphene nanotubes with superior properties for high-performance supercapacitors and efficient oxygen evolution reaction

The rational design of a novel material system with superior properties of energy storage and conversion is a significant work. In this paper, amorphous nickel sulfide nanoparticles anchored on N-doped graphene nanotubes (N-GNTs@NSNs) were firstly synthesized by a facile electrochemical-deposition method, which can serve as free-standing robust supercapacitor electrode materials and electrocatalysts. Stemming from the disordered structure of amorphous active materials and the synergy of novel N-GNT framework materials, the as-prepared N-GNT@NSN electrode unveils prominent capacitive behaviors, including a large specific capacity of 240 mA h g-1 (2160 F g-1), decent rate capability, and outstanding cycling stability (95.8% of capacity retention after 12 000 cycles). An asymmetric supercapacitor with N-GNTs@NSNs as the positive electrode and active carbon (AC) as the negative electrode is further assembled, which shows a maximum energy density of 49.5 W h kg-1 at a power density of 800 W kg-1 and robust stability (96.6% capacity retention after 12 000 cycles). Moreover, the electrode also possesses high activities in the oxygen evolution reaction (OER), namely it can attain a current density of 10 mA cm-2 at an overpotential of 284 mV in 1 M KOH. This finding is not only important for significantly enhancing the electrochemical performances of supercapacitor electrode materials and electrocatalysts, but also lays the solid foundation for their further industrial applications in energy storage and conversion systems.

Fabrication of a Hierarchical Ni(OH)2 @Ni3 S2 /Ni Foam Electrode from a Prussian Blue Analogue-Based Composite with Enhanced Electrochemical Capacitive and Electrocatalytic Properties

Developing high-efficiency, cost-effective, and durable electrodes is significant for electrochemical capacitors and electrocatalysis. Herein, a 3D bifunctional electrode consisting of nickel hydroxide nanosheets@nickel sulfide nanocubes arrays on Ni foam (Ni(OH)₂ @Ni₃ S₂ /NF) obtained from a Prussian blue analogue-based precursor is reported. The 3D higher-order porous structure and synergistic effect of different compositions endow the electrode with large specific surface area, facile ion/electron transport path, and improved conductivity. As a result, the Ni(OH)₂ @Ni₃ S₂ /NF electrode exhibits a high specific capacity of 211 mA h g^-1 at a current density of 1 A g^-1 and 73 % capacity retention after 5000 cycles at 5 A g^-1 . Moreover, the Ni(OH)₂ @Ni₃ S₂ /NF electrode has superior electrocatalytic activity for the hydrogen evolution reaction with low overpotentials of 140 and 210 mV at current densities of 10 and 100 mA cm^-2 , respectively. The synthetic strategy for the unique higher-order porous structure can be extended to fabricate other composite materials for energy storage and conversion.

Hierarchical Co-doped SnS2@Ni(OH)2 double-shell crystalline structure on carbon cloth with gradient pore distribution for superior capacitance

Hierarchical Co-doped SnS₂@Ni(OH)₂ double-shell nanosheet arrays are coated on carbon cloth, the vertically aligned arrays with gradient pore distribution can facilitate the charge/ion transfer rate, thus improve the energy storage performance.

Controlled Hydrolysis of Metal-Organic Frameworks: Hierarchical Ni/Co-Layered Double Hydroxide Microspheres for High-Performance Supercapacitors

Pseudomorphic conversion of metal-organic frameworks (MOFs) enables the fabrication of nanomaterials with well-defined porosities and morphologies for enhanced performances. However, the commonly reported calcination strategy usually requires high temperature to pyrolyze MOF particles and often results in uncontrolled growth of nanomaterials. Herein, we report the controlled alkaline hydrolysis of MOFs to produce layered double hydroxide (LDH) while maintaining the porosity and morphology of MOF particles. The preformed trinuclear M₃(μ₃-OH) (M = Ni²⁺ and Co²⁺) clusters in MOFs were demonstrated to be critical for the pseudomorphic transformation process. An isotopic tracing experiment revealed that the ¹⁸O-labeled M₃(μ₃-¹⁸OH) participated in the structural assembly of LDH, which avoided the leaching of metal cations and the subsequent uncontrolled growth of hydroxides. The resulting LDHs maintain the spherical morphology of MOF templates and possess a hierarchical porous structure with high surface area (BET surface area up to 201 m²·g^-1), which is suitable for supercapacitor applications. As supercapacitor electrodes, the optimized LDH with the Ni:Co molar ratio of 7:3 shows a high specific capacitance (1652 F·g^-1 at 1 A·g^-1) and decent cycling performance, retaining almost 100% after 2000 cycles. Furthermore, the hydrolysis method allows the recycling of organic ligands and large-scale synthesis of LDH materials.

Temperature controlled diffusion of hydroxide ions in 1D channels of Ni-MOF-74 for its complete conformal hydrolysis to hierarchical Ni(OH)2 supercapacitor electrodes

Conformal hydrolysis of MOF precursors is a promising strategy to prepare hierarchical metal hydroxide electrode materials on a large scale with low cost and high efficiency. However, a complete transformation is challenging due to the normal "outside-in" conversion process. After studying the hydrolysis of Ni-MOF-74, which has regular 1D channels, we suggest that the transformation to Ni(OH)2 can occur simultaneously outside and within the precursor depending on the treatment temperature. Molecular dynamics simulations reveal that a higher temperature weakens the steric effects of OH- ions and facilitates the diffusion in the regular channels, and therefore, a complete transformation from Ni-MOF-74 to Ni(OH)2 is achieved. It is for the first time demonstrated that the 1D channels of MOFs are utilized for the complete conformal hydrolysis of Ni-MOF-74 to Ni(OH)2 electrode materials. Meanwhile, we also perform pioneering work illustrating that the complete conformal hydrolysis is the key to the improved supercapacitor performances of the MOF-derived Ni(OH)2 electrodes. The prepared Ni(OH)2 electrode under the optimized conditions has a specific capacity of 713.2 C g-1 at a current density of 1 A g-1, which is at least 28% larger than those of the Ni(OH)2 prepared at other temperatures. The detailed analyses based on CV and EIS of the obtained Ni(OH)2 electrodes indicate that the residual MOFs within electrodes due to incomplete hydrolysis significantly influence the diffusion length and diffusion efficiency of OH-, drastically lowering the supercapacitor performances.

Core–shell assembly of carbon nanofibers and a 2D conductive metal–organic framework as a flexible free-standing membrane for high-performance supercapacitors

A hybrid core–shell material based on carbon nanofibers and a 2D conductive metal–organic framework has been fabricated into a flexible free-standing membrane as an electrode material for supercapacitors.

A Ni(OH)2 nanopetals network for high-performance supercapacitors synthesized by immersing Ni nanofoam in water

Developing a facile and environmentally friendly approach to the synthesis of nanostructured Ni(OH)₂ electrodes for high-performance supercapacitor applications is a great challenge. In this work, we report an extremely simple route to prepare a Ni(OH)₂ nanopetals network by immersing Ni nanofoam in water. A binder-free composite electrode, consisting of Ni(OH)₂ nanopetals network, Ni nanofoam interlayer and Ni-based metallic glass matrix (Ni(OH)₂/Ni-NF/MG) with sandwich structure and good flexibility, was designed and finally achieved. Microstructure and morphology of the Ni(OH)₂ nanopetals were characterized. It is found that the Ni(OH)₂ nanopetals interweave with each other and grow vertically on the surface of Ni nanofoam to form an "ion reservoir", which facilitates the ion diffusion in the electrode reaction. Electrochemical measurements show that the Ni(OH)₂/Ni-NF/MG electrode, after immersion in water for seven days, reveals a high volumetric capacitance of 966.4 F/cm³ at a current density of 0.5 A/cm³. The electrode immersed for five days exhibits an excellent cycling stability (83.7% of the initial capacity after 3000 cycles at a current density of 1 A/cm³). Furthermore, symmetric supercapacitor (SC) devices were assembled using ribbons immersed for seven days and showed a maximum volumetric energy density of ca. 32.7 mWh/cm³ at a power density of 0.8 W/cm³, and of 13.7 mWh/cm³ when the power density was increased to 2 W/cm³. The fully charged SC devices could light up a red LED. The work provides a new idea for the synthesis of nanostructured Ni(OH)₂ by a simple approach and ultra-low cost, which largely extends the prospect of commercial application in flexible or wearable devices.